This invention relates generally to control systems for turbochargers. More particularly, this invention relates to control systems that sense the exhaust gas pressure to control variable geometry turbochargers on internal combustion engines.
Many internal combustion engines use turbochargers to improve engine performance. A turbocharger increases the density of the intake air into the engine. The higher density air increases the amount of fuel the engine may combust. As a result, the power output of the engine increases.
Turbochargers typically have a turbine and a compressor connected by a common shaft. The turbine has blades attached to a wheel, which is mounted on the shaft. A turbine housing encloses the turbine and connects to the exhaust gas manifold of the engine. The turbine housing has vanes for directing the exhaust gases against the turbine blades. The compressor has blades attached to another wheel, which also is mounted on the shaft. A compressor housing encloses the compressor and connects to the intake air manifold of the engine. The compressor housing has vanes for assisting the compressor to pressurize intake air. The compressor housing is isolated from the turbine housing.
In operation, exhaust gases pass through the exhaust gas manifold into the turbine housing. The vanes in the turbine housing direct the exhaust gases against the turbine blades. The exhaust gas pressure causes the turbine to spin, which causes the compressor to spin. The spinning compressor pressurizes the intake air. As a result, higher density air is provided to the intake air manifold.
In a turbocharger, the exhaust gas pressure has a direct effect on the intake air pressure. As the exhaust gas pressure increases, the turbine and consequently the compressor spin faster. A faster spinning compressor increases the intake air pressure. The opposite effect occurs as the exhaust gas pressure decreases.
Many turbochargers have a fixed geometry. The vanes in the turbine and compressor housings are stationery. By design, a fixed-geometry turbocharger operates efficiently at a particular engine speed and load. Conversely, it operates less efficiently at engine speeds and loads for which it is not designed.
At low engine speeds, the exhaust gas pressure is low. It may be below the minimum necessary for operating the turbine. As the engine accelerates from idle or slow speeds, there is a delay from the time when the engine load increases to the time when there is sufficient exhaust gas pressure to spin the turbine. Even when the turbine spins, the exhaust gas pressure may not reach a high enough pressure fast enough to spin the turbine as fast as it is necessary for the compressor to produce the desired intake air pressure.
The exhaust gas pressure increases as engine speed increases. At some point, the pressure becomes high enough to overpower the turbocharger. An overpowered turbocharger reduces engine performance. Additionally, the high exhaust pressure associated with an overpowered turbocharger may cause the turbocharger to fail from fatigue, broken seals, and similar problems.
To improve efficiency, fixed-geometry turbochargers are sized to provide high compressor speeds at low engine speeds. The vanes in the turbine housing usually narrow to increase the exhaust gas pressure. The vanes also direct the exhaust gas flow toward a portion of the turbine blades. While these changes improve the performance of the turbocharger at low engine speeds, they adversely affect the performance of the turbocharger at high engine speeds. The narrowing of the vanes lowers the exhaust gas pressure at which the turbocharger becomes overdriven.
To avoid overdriving, fixed-geometry turbochargers have a waste gate or similar valve positioned between the turbine and the exhaust gas manifold. When the exhaust gas pressure reaches a certain level, the waste gate opens to divert exhaust gases away from the turbine. This approach responds and corrects for an overdriving condition. However, it waits for the condition to occur before responding. It also wastes energy and requires additional equipment.
New turbocharger designs have a variable geometry. The turbine and/or compressor housings have variable nozzles, which move to change the flow area and flow direction. In many designs, only the turbine has variable nozzles.
A variable nozzle turbine (VNT) turbocharger typically has curvilinear nozzles, which rotate between open and closed positions about a pivot. In some designs, the closed position leaves a small gap between the nozzles. In other designs, the nozzles touch when they are closed, which essentially stops the flow of exhaust gas to the turbine. The nozzles connect to each other by a ring or similar apparatus to move in unison. An electronic control module sends an electronic signal to activate a solenoid, pneumatic valve, or similar device.
When the exhaust gas pressure is low, the nozzles close to create a narrower area for the exhaust gases to flow. The narrower area restricts gas flow through the turbine housing, thus increasing exhaust gas pressure. The nozzles also direct the exhaust gases optimally at the tips of the turbine blades. The directed flow and higher pressure enables the turbine to start spinning sooner and at a faster rate. As a result, a VNT turbocharger provides the high compressor speeds desired at low engine speeds.
As the exhaust gas pressure increases, the nozzles open to reduce the restriction to the gas flow. The gas flow also is directed toward the entire length of the turbine blades. With less restriction and broader gas flow, the turbine and consequently the compressor spins slower than if the nozzles were closed under these conditions. As a result, the turbocharger is able to respond and correct for overdriven conditions.
Proper nozzle control is necessary to optimize performance of a VNT turbocharger. Internal combustion engines, especially those in vehicles, have constantly changing demands. One moment, the engine is at low speed. The next moment, the engine is at high speed. Engine load and other parameters change almost constantly. Accordingly, the nozzles must adjust to new operating conditions quickly. If the nozzles delay closing, such as when the engine goes from high to low speeds, the turbocharger will not provide the desired intake air pressure. If the nozzles delay opening, such as when the engine goes from low to high speeds, the turbocharger will be overdriven.
In most designs, VNT turbochargers are controlled by the intake air pressure. The measured intake air pressure is compared to a desired intake air pressure. A sensor is located in the intake air manifold to determine the measured intake air pressure. The engine""s electronic control module (ECM) or other microprocessor determines the desired intake air pressure based on engine operating parameters such as engine speed, engine load, ambient air pressure, etc. If the measured intake pressure is higher then the desired intake pressure, the ECM opens the nozzles until the measured and desired intake pressures are equal. Conversely, if the measured intake pressure is lower than the desired intake pressure, the ECM closes the nozzles until the intake pressures are equal.
To open or close nozzles, the ECM sends an electric signal to the solenoid, pneumatic valve, or other device controlling the nozzles. The strength of the electric signal or duty cycle determines the position of the nozzles. The duty cycle is a percentage of the total electrical signal necessary to move the nozzles into their closed position. While the duty cycle is indicative of the nozzle position, the duty cycle for a particular nozzle position varies from turbocharger to turbocharger.
Intake air pressure is not suitable for optimizing the performance of a VNT turbocharger. Generally, the intake air pressure increases as the nozzles close. However, there is position where the intake air pressure reaches a maximum level and then decreases if the nozzles close further.
FIG. 1 shows the relationship between the intake air pressure and the turbine duty cycle (nozzle position). As the turbine duty cycle increases from 20 to 60 percent, the intake air pressure increases from 7 to 28 in. Hg. As the turbine duty cycle increases above 60 percent, the intake air pressure decreases. The nozzles have restricted the flow of gases to the turbine sufficiently to slow the compressor. Consequently, the intake air pressure decreases to 19 in. Hg at a duty cycle of 80 percent. At this point and beyond, the nozzles are closed.
As the nozzles close beyond the position of maximum air intake pressure, they prevent exhaust gases from flowing across the turbine. The turbine and compressor turn slower with less exhaust gas flow. However, the exhaust gas pressure increases dramatically. This combination of a slower compressor and higher exhaust gas pressure decreases the engine torque and increases fuel consumption. The turbocharger is providing excess exhaust pressure to the engine. The excess exhaust pressure effectively xe2x80x9cstealsxe2x80x9d work from the engine to produce the high exhaust gas pressure. It turns the engine into an air compressor, thus diverting power from the transmission.
The maximum intake air pressure is dependent largely upon the exhaust gas volume. At lower engine speeds, the maximum intake air pressure occurs at higher duty cycles (the nozzles are more closed). At higher engine speeds, the maximum intake air pressure occurs at lower duty cycles (the nozzles are more open). This affect is more noticeable on VNT turbochargers where the nozzles close completely.
It is difficult to control a VNT turbocharger based on the intake air pressure. At many intake air pressures, the ECM cannot properly determine whether to open or close the nozzles. For example in FIG. 1, an intake air pressure of 25 in. Hg occurs at two duty cycles. Depending on the duty cycle, opening the nozzles may decrease or increase the intake air pressure. Similarly, closing the nozzles also may decrease or increase the intake air pressure. The problem worsens if the turbocharger has nozzles that close completely.
In addition to control problems, controllers based on the intake air pressure do not identify and address the excessive exhaust gas pressure conditions when the turbocharger may be overdriven. These conditions may occur prior to the intake air pressure reaching a maximum.
To address excessive exhaust gas pressure, some turbochargers include an exhaust gas pressure sensor in the exhaust gas manifold. In one approach, the ECM opens the nozzles when the exhaust gas pressure reaches a certain level. The ECM keeps opening the nozzles until the exhaust gas pressure returns to a proper level. Another design compares the intake air pressure with the exhaust gas pressure. When the difference between the pressures reaches a certain level, the ECM opens the nozzles until the difference returns to a proper level.
While these approaches respond to excessive exhaust gas pressure, they do so after the overdriving conditions already exist. They also require additional equipment, namely a sensor and associated control interfaces. In addition, they create a xe2x80x9cseesawxe2x80x9d effect when operating the turbocharger. When the intake air pressure is lower than the desired intake air pressure, the ECM closes the nozzles. This action increases the exhaust gas pressure to drive the turbine and compressor faster. When the exhaust gas pressure exceeds a certain level, the ECM opens the nozzles to reduce the exhaust gas pressure. At that point, if the measured intake air pressure is below the desired intake air pressure, the ECM closes the nozzles to increase the intake air pressure. This seesaw effect continues until the operating parameters of the engine change.
In another design, a VNT turbocharger is controlled by sensing the position of the variable vanes. A predetermined map provides a desired vane position based upon engine conditions such as engine speed and load. As these engine conditions change, the variable vanes are moved to the desired vane position for those conditions. Theoretically, the desired vane position should provide the desired intake boost pressure. However, the vane position does not adequately adjust for the variability in exhaust gas volume and pressure associated with changing engine conditions. In addition, the vane position to intake boost pressure relationship will have errors unless manufacturing tolerances are small between turbochargers.
Accordingly, there is a need for a turbocharger control system that maximizes the available intake boost pressure while avoiding excessive exhaust gas pressure and overdriving conditions under variable and changing engine operations.
The present invention provides a system and method for using an engine""s exhaust back pressure to control a variable geometry turbocharger. The control system determines a desired exhaust back pressure based on engine speed and engine load. The desired exhaust back pressure is compared with a measured exhaust back pressure to determine the difference between the measured and desired exhaust back pressures. The difference between the desired and measured pressures is used to determine the duty cycle for the turbocharger.
The exhaust back pressure provides greater controllability over the prior art. This enhanced controllability enables additional embodiments for controlling turbochargers with cold weather warm-up, engine braking, and exhaust gas recirculation (EGR) capabilities. In cold weather, xe2x80x9cextraxe2x80x9d exhaust pressure will cause the engine to increase fuel consumption thus shortening the time to warm-up of the engine.
During braking, the engine may be used to slow the vehicle. Higher exhaust gas pressures increase negative torque and thus slow the engine. The decrease in engine speed slows the vehicle when the transmission is engaged. Engine braking is desirous to augment cruise control. For engines with EGR, the control system ensures the exhaust gas pressure is always higher than the intake air pressure. This enables the exhaust gas to enter the intake air manifold as desired. It also avoids additional equipment associated with EGR.
While these embodiments use the exhaust back pressure to determine the duty cycle for the turbocharger, an alternate embodiment uses the exhaust gas pressure to adjust the duty cycle determined by other operating parameters. In the alternate embodiment, a base duty cycle is determined from the engine speed and the engine load. The difference between the measured and desired exhaust back pressures is used to determine an exhaust pressure control duty cycle. The base duty cycle is then adjusted by the an exhaust pressure control duty cycle to provide an adjusted duty cycle to the turbocharger.
The following drawings and description set forth additional advantages and benefits of the invention. More advantages and benefits are obvious from the description and may be learned by practice of the invention.